In a typical proportional navigation guidance system for an air vehicle such as a guided missile, the magnitude and direction of the missile-to-target line-of-sight rate of change is normally measured by a receptor, often referred to as an antenna or a seeker, and this output is used to control the rate of change of the missile velocity in both magnitude and direction through the guidance/control section. The primary measurement performed by the seeker antenna is the error angle between the missile-to-target line-of-sight and the antenna electrical boresight.
In a body gimballed sensor system, the angle sensed by the antenna leads to an output which is proportional not only to the desired missile-to-target line-of-sight rate of change but also to missile body turning rate. A gimballed antenna cannot be used to produce an inertial rate such as a line-of-sight rate without the summation of the body rate occurring in the seeker. This is because a body gimballed sensor is not inertially referenced. If this seeker signal, which includes information proportional to the missile body turning rate, is fed to the missile control system, a closed loop is created. This loop, commonly referred to as a body coupling loop, is normally undesirable in that it will adversely affect the stability and guidance accuracy of the missile.
In vehicles which utilize a body referenced or body gimballed sensor it has been suggested to either internally or externally decouple body motion in order to achieve the desired line-of-sight rate with respect to space. Externally decoupling the body rate is normally achieved by summing the body rate with the receiver output, that is, the beta dot command (.beta..sub.c) signal. A drawback of such external decoupling is that receiver errors (gain, phase, quadrature errors) degrade the coupling match. Because the externally decoupled body rate signal must be dynamically matched to the closed loop seeker response and receiver errors change the closed loop response, the need is created for very tight tolerances on the receiver errors. This translates into time and money.
Internal decoupling is accomplished by summing the body rate with the signal out of the receiver to produce a beta dot command signal. Internal decoupling transforms the body gimballed sensor into a surrogate space stabilized sensor. The problem that arises with a surrogate space stabilized sensor is that drifts and biases within the servo rate loop, which positions the antenna, cause a bias in the line-of-sight rate and such biases can degrade performance. Again, to eliminate problems of this type, expensive hardware must be used within the servo rate loop to minimize the biases.
Space stabilized, inertially referenced sensors and body fixed antenna systems are not applicable to the aforementioned decoupling system. By definition, a space stabilized sensor is intrinsically decoupled from body motion. Body fixed sensors are rigidly fixed to the vehicle and therefore cannot be commanded to move to compensate for vehicle body motion. Hence, internal decoupling cannot be utilized for either system.